1. Field of the Invention
The present invention relates to a single-focus lens, for example, used as an imaging lens in an electronic camera (digital camera).
2. Description of the Related Art
In recent years, an electronic camera which images a subject using a solid-state image sensor such as a charge-coupled device (CCD) in place of a silver-salt film has become popular. Generally, the electronic camera falls into two types: still camera and video camera. The still camera captures still pictures, whereas the video camera captures motion pictures. Imaging lenses described in Japanese Unexamined Patent Application Publication No. Hei 8-152555 and No. Hei 9-90213 are examples of conventional imaging lenses used in such electronic cameras. Japanese Unexamined Patent Application Publication No. Hei 8-152555 is directed to an imaging lens having six glass lenses. Japanese Unexamined Patent Application Publication No. Hei 9-90213 is directed to an imaging lens which comprises five lenses: four glass lenses and one plastic aspheric lens.
Now miniaturization of the electronic camera is in demand as is the camera using silver-salt films. Therefore, desirably the imaging lens for use in the electronic camera is simply constituted and downsized, having a shorter overall length. Further, desirably the imaging lens is manufactured at a low cost. Conventional imaging lenses, however, do not fully satisfy these requirements. For example, the imaging lens described in Japanese Unexamined Patent Application Publication No. Hei 8-152555 has six glass lenses. Six lenses are rather too many and the use of glass for all lenses results in high manufacturing costs. The imaging lens described in Japanese Unexamined Patent Application Publication No. Hei 9-90213 has five lenses and one of these lenses is made of plastic with the aim of cost reduction. However, its overall length is long, thus fails in downsizing.
In order to achieve reduction in the number of lenses and overall length, proposed is an imaging lens having a plurality of aspheric lenses. In this case, desirably the aspheric lenses are made of optical plastic (plastic) rather than glass in view of costs and productivity. With plastic lenses, a change in optical performance due to environmental changes such as a change in temperature or humidity is large as compared to that with glass lenses, so that it is preferred that its power be set small in order to reduce the change in optical performance. Thus, when a plurality of aspheric surfaces is used in the imaging lens, desirably the lenses are made of plastic and power is distributed appropriately considering environmental changes.
This applicant proposed a single-focus lens which exhibits preferred optical performance with a shorter overall length and a simple configuration and, is fabricated at a low cost in U.S. patent application Ser. No. 09/790,039 (Japanese Unexamined Patent Application Publication No. 2000-062991), still pending. This single-focus lens is constituted of a total of five lens elements and two lens elements thereof are plastic aspheric lenses. However, part of the lens elements of this single-focus lens is susceptible to improvement in view of fabrication process. For example, in the single-focus lens set forth in an experimental example, the fourth lens L4 made of glass is rather thick and its radius of curvature is small, which requires relatively complicated spherical surface fabrication technique.
Although the single-focus lens proposed in U.S. patent application Ser. No. 09/790,039 (Japanese Unexamined Patent Application Publication No. 2000-062991) comprises 5 lenses so that cost reduction and simple configuration are realized, the imaging lens for the electric camera requires further cost reduction and simple configuration. Imaging lenses of the known art have never fully satisfied requirements such as appropriate power distribution, simple configuration, cost reduction and shorter overall length as a whole. Particularly, the imaging lenses of the known art are susceptible to improvement in view of cost reduction.
The present invention has been achieved in view of the above problems and its first purpose is to provide a single-focus lens with a shorter overall length which exhibits optimal optical performance chiefly for imaging lenses and does not require complicated fabrication technique. Its second purpose is to provide a single-focus lens which exhibits preferable optical performance chiefly for imaging lenses and is fabricated at very low cost with a very simple configuration and a shorter overall length.
According to the single-focus lens of a first aspect of the invention, consecutively arranged from an object side are a first lens having a meniscus shape and a positive refractive power, the first lens directing its concave to the object side; a second lens having a meniscus shape and including at least one aspheric surface; a third lens directing its concave to the object side and having a negative refractive power; a fourth lens directing its convex to an image side and having a positive refractive power; a fifth lens directing its convex to the image side and having a positive refractive power; and a sixth lens having a meniscus shape, including at least one aspheric surface, and directing its concave to the image side.
According to the single-focus lens of the first aspect of the invention, principally in view of productivity of the aspheric surface, preferably the second lens and the sixth lens are made of optical plastic.
According to the single-focus lens of the first aspect of the invention, as for the second lens L2 and the sixth lens L6 which are made of optical plastic, principally in order to limit refractive power and suppress deterioration in optical performance due to environmental changes, the following conditional expressions (1) and (2) are satisfied: (1) xe2x88x920.2 less than f/f2 less than 0.2 and (2) xe2x88x920.2 less than f/f6 less than 0.2, where the f represents an overall focal length; the f2 represents a focal length of the second lens; and the f6 represents a focal length of the sixth lens.
According to the single-focus lens of the first aspect of the invention, chiefly for the purpose of correcting chromatic aberration, the following conditional expressions (3), (4), (5) and (6) are satisfied: (3) 1.70 less than Nd1, (4) 35  greater than xcexdd1, (5) 35 greater than xcexdd3, and (6) 35 less than xcexdd4, where the Nd1 represents refractive index of the first lens for a wavelength of d-line, the xcexdd1 represents Abbe number of the first lens for the wavelength of d-line, the xcexdd3 represents Abbe number of the third lens for the wavelength of d-line, and the xcexdd4 represents Abbe number of the fourth lens of the wavelength of d-line.
According to the single-focus lens of the first aspect of the invention, a plurality of aspheric lenses is employed, so that the single-focus lens can exhibit preferable optical performance with a shorter overall length and a relatively small number of lenses. Two lenses (the fourth lens and the fifth lens) similar in shape are arranged on the object side of the sixth lens. Accordingly, positive refractive power is easily divided between the two lenses.
According to the single-focus lens of a second aspect of the invention, consecutively arranged from an object side are a first lens having a meniscus shape and a positive refractive power; a second lens having a meniscus shape and at least one aspheric surface; a third lens directing its convex to an image side and having a positive refractive power; and a fourth lens having a meniscus shape, including one aspheric surface and directing its concave to the image side.
According to the single-focus lens of the second aspect of the invention, principally in view of productivity of the aspheric surface, preferably the second lens and the fourth lens are made of optical plastic.
According to the single-focus lens of the second aspect of the invention, as for the second lens L2 and the fourth lens L4 which are made of optical plastic, principally in order to limit refractive power and suppress deterioration in optical performance due to environmental changes, the following conditional expressions (11) and (12) are satisfied: (11) xe2x88x920.2 less than f/f2 less than 0.2 and (12) xe2x88x920.2 less than f/f4 less than 0.2, where the f represents an overall focal length; the f2 represents a focal length of the second lens; and the f4 represents a focal length of the fourth lens.
According to the single-focus lens of the second aspect of the invention, chiefly for the purpose of correcting chromatic aberration, the following conditional expressions (13), (14) and (15) are satisfied: (13) 1.70 less than Nd1, (14) 35 greater than xcexdd1, and (15) 50 less than xcexdd3 where the Nd1 represents refractive index of the first lens for a wavelength of d-line, the xcexdd1 represents Abbe number of the first lens for the wavelength of d-line and the xcexdd3 represents Abbe number of the third lens for the wavelength of d-line.
According to the single-focus lens of the second aspect of the invention, a plurality of aspheric lenses is employed, so that the single-focus lens can exhibit preferable optical performance with a shorter overall length and a very small number of lenses.
Other and further objects, features and advantages of the invention will appear more fully from the following description.